1. Field of the Invention:
This invention relates to zoom lenses and, more particularly, zoom lenses of increased aperture ratio to an F-number of 1.2 with a high zoom ratio of about 10, while still preserving good optical performance over the entire zooming range, suited to photographic cameras or video cameras.
2. Description of the Related Art:
For the photographic camera or video camera there has been demand for zoom lenses having a large aperture ratio and a high zoom ratio and yet having a high optical performance. Of these, the zoom lens for the video camera for home use is required to have a high resolving power of, for example, 500 lines/mm in spatial frequency over the entire area of the picture frame, as the number of resolving cells in the image sensor such as CCD increases and the recording method improves to the S-VHS or the like.
Besides this, because, in the commonly available video cameras, the image sensor has relatively low sensitivity, the aperture ratio of the zoom lens must be as large as possible.
It is known to provide a zoom lens whose F-number is on the order of 1.4 and whose zooming range is on the order of 10 as, for example, disclosed in Japanese Laid-Open Patent Applications Nos. Sho 54-17042 and 54-23556. In these documents, there has been made a proposal for using five lens groups, or a so-called five component type comprising, from front to rear, a first lens group of positive power for focusing, a second lens group of negative power for varying the image magnification, a third lens group for compensating for the image shift resulting from the variation of the image magnification, a fourth lens group for making afocal the light leaving from the third lens group, and a fifth lens group for forming an object image.
In general, to achieve great increases of the aperture ratio and the zooming range at once in such a manner that high optical performance is preserved, it is of great value to increase the number of lens elements or to weaken the refractive power of each lens group. These techniques of using so many lens elements and decreasing the refractive powers of all the lens groups have, however, to increase the physical length and the lens diameter, and, therefore, are not very suited to be applied to the zoom lens for the handy video camera, as it is of great importance to minimize its bulk and size and the weight.
Also, with the F-number, if increased, the lens diameter is advantageously minimized. But such a zoom lens is not very suited to the video camera using the image sensor of low sensitivity.
The related art to the present invention is disclosed in U.S. Pat. Nos. 4,812,024 and 4,832,471 and U.S. patent applications Ser. No. 156,935 filed on Feb. 17, 1988 now U.S. Pat. No. 4,846,563, Ser. No. 301,951 filed on Jan. 26, 1989, Ser. No. 223,108 filed on Jul. 22, 1988 and Ser. No. 355,348 filed on May 23, 1989.